All of these techniques accomplish the same thing - they confine knowledge
of implementation details to the smallest possible part of a program. That
is, they keep a secret of some sort.

Constant and liberal use of the above techniques is recommended.

An interesting quote from chapter one of Design Patterns, regarding
the use of generic references:

"This so greatly reduces implementation dependencies between subsystems
that it leads to the following principle of reusable object-oriented design
:

Program to an interface, not an implementation.

Don't declare variables to be instances of particular concrete classes.
Instead, commit only to an interface defined by an abstract class. You
will find this to be a common theme of the design patterns in this book."

(They state a second general principle as well: "Favor object composition
over class inheritance.")